1. Field of the Invention
The present invention relates to measuring and mapping the electrical resistivity of the earth's subsurface with measurements from transmission of electrical energy from wells in subsurface reservoirs to remote electrodes.
2. Description of the Related Art
Electromagnetic (EM) soundings probe electrical resistivity (or its inverse, conductivity) as a function of lateral position and depth in the earth. Resistivity data from EM soundings are obtained in surveys over regions of interest and used to obtain information about subsurface geological structures and/or man-made objects of interest. These include, for example, mineral deposits, hydrocarbon reservoirs, Enhanced-Oil-Recovery/Improved-Oil-Recovery injected fluids and in-situ fluids, hydrofracturing injected fluids and slurries, groundwater reservoirs, fluid fronts, contaminants, permafrost, weathered layers, infrastructure, tunnels, and underground facilities. Since the resistivities of such objects and the surrounding media are generally quite dissimilar, they can, in theory, be discriminated by means of measurement of the subsurface resistivity. Using this methodology, the depth, thickness, and lateral extent of objects of interest can be determined, depending on the capabilities of naturally occurring EM sources, or controlled-source EM sources such as a transmitter.
A number of measurement scenarios for sounding have been employed in the past, including natural and/or controlled electric and/or magnetic sources with many different source and/or receiver combinations and/or geometries for surface-based configurations, borehole-to-surface configurations, surface-to-borehole configurations, single borehole configurations, and multiple borehole (e.g., cross-borehole) configurations. Recently, the borehole-to-surface configuration, with a controlled electromagnetic field source at a desired depth in a borehole and an array of electromagnetic receivers at the ground surface, has been demonstrated to have adequate sensitivity to map the boundary of a hydrocarbon reservoir to a distance of 2 km-3 km from a borehole.
Controlled-source EM methods include both frequency-domain and time-domain measurements of the fields in response to artificially generated EM fields. In time-domain EM surveys routinely practiced by industry, an antenna measures magnetic fields generated from subsurface currents induced in the earth. In induced polarization (IP) or spectral induced polarization (SIP) surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields generated from subsurface currents induced in the earth. In magnetic induced polarization (MIP) or magnetometric resistivity (MMR) or sub-audio magnetic (SAM) surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields generated from subsurface currents induced in the earth. In controlled-source audio-magnetotelluric (CSAMT) or controlled-source magnetotelluric (CSMT) surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields and an array of receiver magnetometers measures magnetic fields generated from subsurface currents induced in the earth. In all of these methods the currents are induced in the earth by a time-varying electromagnetic field. When the source is located in a well borehole and sensors or receivers are at the surface, these types of surveys are known as borehole to surface electromagnetic or BSEM surveys.
In BSEM surveys, each EM receiver measures the EM field at the ground surface produced by the EM source at depth in the borehole. The source field propagates through the earth in a manner that depends on the electrical resistivity distribution within the earth. Measurements of the EM field at multiple points on the surface can be processed using a number of conventional methods, known in the art, to produce a three-dimensional map of the resistivity distribution in the subsurface region covered by the receiver array.
Present BSEM surveys employ an array of 1000 or more measurement points. For electric field receivers, two electrodes placed 10 m to 100 m apart are needed for each axis of measurement. Installing such electrodes requires accurate geolocation of 2000 or more physical sites for a single measurement axis, for example the electric field radial to the borehole, and further geolocation is necessary when additional axes, for example the tangential field, are desired. Furthermore, conventional electrodes used for electric field surveys require burial at depths of order 50 cm. For magnetic field receivers only a single unit is needed for each axis of measurement, but the hole required for burial is 3 to 5 times larger than needed for an electrode used for electric field sensing. Recently, an electric field sensor that does not require burial, but can simply be placed on the surface of the ground, has been introduced. However, two sensors are still required for each axis of measurement, and each sensor requires accurate geolocation.
Accurate geolocation is required to establish the position of each sensor within the sensor array and also to ensure correct alignment of the sensor axes. An absolute position accuracy of 1 m results in a location error of order 0.1% in an array of scale 1 km. However over a 50 m separation between electrodes, a position error of 1 m results in an angular error of order 5%, which in turn results in an error of order 5% in measuring the electric field.
A second source of measurement error is a variation in the electrical contact impedance between the electrode and the earth. For example a change of 10 kΩ in the contact impedance results in a change in the measured signal of order 0.1%. A third source of error in measuring the E-field in the ground is the static shift effect that is due to inhomogeneities in the ground conductivity that occur very close to the sensor. Fourthly, the signal at the ground surface can be affected by the local roughness of the ground and variations in elevation. To address these measurement issues, careful, time consuming installation of the EM sensors is required.
Even when the best installation methods have been employed, the four sources of error associated with the receivers have in general limited the accuracy of the field measured at any point. In particular, if a BSEM survey is repeated at a later time at the same site, then variations in installing the sensors are likely to be the limiting factor in the determining if differences in the subsurface have developed in the time between the two surveys. Permanent installation of sensors addresses geolocation and alignment issues, but is still vulnerable to changes in coupling, static shift and surface features that may occur due to changes in weather conditions.
The accuracy problems mentioned above have significantly affected the time needed to install a surface sensor array. A further factor in the time needed to complete a BSEM survey is the local EM interference at the site. This interference, also known as cultural noise, is usually man-made and is produced by power lines and electrical equipment in the vicinity of the survey location. The cultural noise can vary across a survey array making it difficult to remove it by processing in the final data.
The installation and interference issues in part set the limits of present EM methods in geophysics. By virtue of the source being located below ground, it becomes possible to consider applying BSEM to deeper subsurface formations with higher spatial resolution than are targeted by traditional EM soundings that use a surface source. However, as a result the requirements for receiver accuracy are increased over those for traditional EM surveys.
In addition to the time needed to install the receivers at the surface, there is the time needed to install the source within the borehole. This is a particular concern for a borehole that is utilized as part of a producing well, because accessing such a well requires that it be removed from production for the duration of a conventional BSEM survey. In addition, because electrical power is applied into the borehole, a safety analysis is required to control the risk of fire and other safety hazards. Such a safety analysis can require several months to obtain approval.